1. Field of The Invention
This invention relates to a method for making an optical recording medium for high-density recording, and more particularly, to a method for making an optical disk of the read-only, write-once or rewriteable type. The invention also relates to such an optical recording medium as mentioned above.
2. Description of the Prior Art
In recent years, read-only optical disks, in which information is recorded in high density, write-once optical disks capable of recording information only once, and rewriteable magnetooptical (MO) disks or phase change (PC) disks, have started to rapidly come into wide use, not to mention compact disks. Among them, those optical disks, which comprise a recording layer containing organic materials, have been extensively developed and now put into practical use.
Organic compounds are more versatile in kind than metals or inorganic compounds, and enable a diversity of functions to be developed by structurally designing same at a molecular level, making it easy to realize high-density recording and high sensitivity. For instance, as a write-once CD (CD-R), there is provided an inexpensive optical disk of high productivity, whose recording layer is formed by spin-coating an organic dye, dissolved in solvent, on a transparent resin substrate having a spiral, continuous groove cut therein.
Further, studies have been made on so-called xe2x80x9csuperresolutionxe2x80x9d optical disks, wherein an optically transparent substrate, in which information is encoded as pits beforehand, is formed thereon with a mask layer containing a reversible dye such as a thermochromic or photochromic compound.
In the optical disk formed with the mask layer, the nonlinear change of light transmittance against light intensity inherent to reversible dyes, such as thermochromic or photochromic compounds, are utilized in a manner as follows. The spot size of a laser beam, which is restricted depending the wavelength of an irradiated laser beam and the numerical aperture (NA) of a pickup lens, can be made smaller than that of the irradiated beam by causing the laser beam to be transmitted only at an intense region of a beam intensity distribution thereof. This enables smaller pits to be reproduced, and as a result, higher densification can be achieved.
As is known in the art, a layer containing such an organic compound as mentioned above has been formed according to the aforesaid spin-coating technique. In a write-once optical disk making use of an organic dye in the recording layer, such as CD-R, usual practice is to form a recording layer by spin-coating an organic dye, dissolved in a solvent, onto a transparent resin substrate having a spiral, continuous groove therein. In this connection, however, an optical disk of next generation, which is so designed as to record in a density higher than conventional optical disks, such as a compact disk, the size and depth of pits and the dimensions such as of a width and depth of the continuous groove tend to become smaller. When a solution of a dye dissolved in solvent is spin-coated onto a substrate formed with such small and shallow pits and such a fine and shallow groove therein, the pits and the groove are filled up with the coated dye, thereby presenting the problem of causing signal characteristics to be degraded owing to the difficulty in tracking the groove and pits at the time of recording and reproduction.
From the standpoint of costs and mass-productivity, most of the recent optical disks make use, as a substrate, of non-crystalline, light-transmitting thermoplastic polymer materials such as polycarbonates. However, the non-crystalline thermoplastic polymer materials, in most cases, exhibit a relatively low resistance to organic solvents. This causes the following problems.
(1) The types of organic solvents, which are usable for the formation of an organic matter-containing layer on the substrate by spin coating and which do not attack the substrate, are limited.
(2) The types of organic matters, such as dyes, to be dissolved in such organic solvents incapable of attacking the substrate are more restricted.
On the other hand, aside from the spin-coating method of forming an organic compound-containing layer, a vacuum deposition method has been studied in order to provide optical disks. For instance, according to Japanese Laid-open Patent Application No. 7-18693,there is disclosed a so-called xe2x80x9csuperresolutionxe2x80x9d optical disk wherein its mask layer having a reversible dye, such as a thermochromic dye, is formed by vacuum deposition under heat. The formation, by vacuum deposition under heat, of such a mask layer as mentioned above on a substrate having small pits therein as formed in a xe2x80x9csuperresolution optical diskxe2x80x9d disenables the pits to be filled up as experienced in the spin-coating method, realizing a high-quality superresolution optical disk.
However, the vacuum deposition under heat has limitations described below and are not always satisfactory in this regard.
(1) Organic compounds, which can be stably deposited, are limited in type. For instance, since organic ionic dyes such as cyanine dyes and polymer compounds do not exhibit any sublimation, they cannot be formed as a film according to the vacuum deposition under heat.
(2) Even if organic compounds used have sublimating properties, they are ordinarily more decomposable than inorganic materials on heating, so that the film-forming speed cannot increase so much, and it undesirably takes a relatively long time for the film formation.
(3) It is difficult to carry out deposition from a single target obtained by mixing a plurality of types of organic compounds while controlling the sublimation speeds of the respective compounds. For the simultaneous vacuum deposition of a plurality of types of organic compounds, a vacuum deposition source for every organic compound has to be provided.
In order to overcome the drawbacks of the vacuum decomposition methods, spraying methods have been proposed, for example, in Japanese Laid-open patent Application Nos. 6-306181 and 7-252671 and also by T. Hiraga et al (J. Vac. Sci. Technol., A12(3), pp. 876-878 (1994) ) and by T. Hiraga et al (Jpn. J. Appl. Phys., 33(9A), 5051-5059 (1994)).
In the methods of the Japanese references, it is stated that a substrate is heated to a temperature not exceeding a thermal decomposition temperature of an organic deposit to remove volatile matter therefrom. The substrate is heated by means of a heater in contact therewith. Under conditions where the substrate placed in a high-vacuum chamber is heated to a temperature lower than the thermal deformation temperature of the substrate, e.g. 150xc2x0 C., if a sprayed mist arrived at the substrate has a large amount of a solvent left therein (e.g. 50wt %), the substrate is rapidly cooled owing to the heat of vaporization of the solvent. When the substrate is made, for example, of a metal or glass having high thermal conductivity, the heat energy corresponding to the heat of vaporization can be immediately supplied from the heater to the substrate. However, where an organic material, e.g. polycarbonate, having low thermal conductivity is used as the substrate, the surface temperature of the substrate considerably lowers, requiring a long time before the vaporization of the solvent left in the deposit on the surface.
We have experimentally confirmed that the above methods are applied to a substrate, which has been encoded with information in the form of pits beforehand or which has a groove or grooves to form a mask layer on the substrate, the pits and/or groove is significantly deformed. More particularly, a thin film of an organic material cannot be formed while keeping the pits and/or groove substantially in original form. Presumably , this is considered due to the fact that when an organic polymer material is used as the substrate, an organic solvent left in contact with the substrate deforms the surface profile of the substrate, and that the deposit containing a large amount of a solvent is caused, more or less, to flow owing to the presence of the solvent before the vaporization thereof, thus making a relatively flat surface. Anyway, the pits and/or groove in the substrate are appreciably deformed or, in the worst case, broken. In the case where glass or other types of materials resistant to solvent are used as a substrate encoded with information, the pits and/or groove may not be reproduced reliably by coverage with an organic deposit.
In the Hiraga et al reports, a spray nozzle heater is used. This heater may be effective, to an extent, in preventing a solvent from being solidified when the temperature lowers owing to the vaporization of the solvent. However, it is not believed that a heat energy necessary for complete vaporization of a solvent is applicable, from outside, to a solution or dispersion being sprayed.
Physical properties of ordinary organic solvents are shown in Tables 1 and 2 below.
In the above Tables, Tvp is a value of an xe2x80x9cimaginary temperature drop accompanied by vaporizationxe2x80x9d which is obtained by dividing heat of vaporization (joules/mole) by specific heat (joule/mol/K) on the assumption that the specific heat of a solvent and the heat of vaporization are, respectively, a constant in a temperature range of from its melting point to boiling point. In practice, the heat of vaporization increases and the specific heat decreases along with the temperature drop of solvent, so that it is considered that an actual temperature drop is greater than the imaginary temperature drop. As will be seen from Tables 1 and 2,the values of Tvp of a number of solvents are indicated. Most of the solvents have the Tvp values which individually exceed the differences in temperature between the melting and boiling points, or the values of Tv-m in the tables. This means that if droplets of a solvent at a boiling point at normal pressures is placed in vacuum, the solvent suffers a temperature drop to its melting point before complete vaporization of the solvent and is solidified. The xe2x80x9csublimationxe2x80x9d of the resultant solid proceeds much slower than vaporization. The heating of the spray nozzle fails to prevent information pits in a substrate from being deformed or broken.
It is accordingly an object of the invention to provide a method for making an optical recording medium which overcomes the problems involved in prior art counterparts.
It is another object of the invention to provide a method for making an optical recording medium wherein a substrate, which has information pits and/or a guide groove therein, is formed thereon with a thin film by spraying, in vacuum, a mist of a solution or dispersion containing an organic compound while causing a solvent to be evaporated from the mist to an extent as much as possible during the time before the mist arrives at the substrate surface whereby the mist arrived at the substrate surface deposits along the surface profile of the substrate without permitting any further flow of the deposit.
It is a further object of the invention to provide a method for making an optical recording medium wherein a substrate, which has information pits and/or a guide groove therein, is formed therein with a thin film by spraying, in vacuum, a solution or dispersion containing an organic compound in the form of a mist while causing a solvent to be evaporated from the mist to an extent as much as possible during the time before the mist arrives at the substrate surface whereby the information pits and/or a guide groove is prevented from deforming or breaking by dissolving the substrate with the solvent.
It is another object of the invention to provide an optical recording medium obtained by the method mentioned above.
The above objects can be achieved, according to the invention, by a method which comprises providing a substrate which is encoded with information in the form of pits and/or a continuous groove beforehand at least on one side thereof, spraying a solution or dispersion of a film-forming material containing at least one organic compound having optical functionality onto the encoded side of the substrate in a vacuum chamber under conditions sufficient to permit a thin film to be formed thereon in a substantially solvent-free condition, and drying the thin film.
The substrate used should preferably be made of a light-transmitting material such as a light-transmitting thermoplastic polymer. It should be noted that the term xe2x80x9csubstantially solvent-free conditionxe2x80x9d means one where substantially all of the solvent is not present in sprayed mist or droplets of the solution or dispersion at the time when the mist contacts with the side of the substrate being sprayed, but such a solvent may be left in amounts not adversely affecting the substrate and the like.